Question

Higher temperature increases the rate of leaching in a solid-liquid system due to:

• A. Increased liquid viscosity and decreased diffusivity
• B. Increased liquid viscosity and increased diffusivity
• C. Decreased liquid viscosity and decreased diffusivity
• D. Decreased liquid viscosity and increased diffusivity

Hint:

Remember that higher temperature generally speeds up most chemical and physical processes involving molecular movement. Think about how heating a liquid makes it "thinner" (less viscous) and how molecules spread out faster in a warm environment (higher diffusivity). Both of these help in dissolving and transporting the solute during leaching.

Answer 1

The Correct Option is D

Step 1: Understand the process of leaching in a solid-liquid system.
Leaching is a mass transfer process where a soluble component (solute) is selectively dissolved from a solid mixture by a liquid solvent. The rate of leaching is influenced by several factors, including temperature, agitation, solvent properties, and the nature of the solid. Step 2: Analyze the effect of temperature on liquid viscosity.
For most liquids, as the temperature increases, the kinetic energy of the liquid molecules increases, allowing them to overcome intermolecular forces more easily. This results in a decrease in the liquid's resistance to flow, which is its viscosity. Therefore, higher temperature generally leads to decreased liquid viscosity. Step 3: Analyze the effect of temperature on diffusivity.
Diffusivity is a measure of the rate at which molecules move from a region of high concentration to a region of low concentration due to random molecular motion. According to the kinetic theory of matter, higher temperature increases the average kinetic energy of molecules, causing them to move faster and collide more frequently. This increased molecular motion leads to a higher rate of diffusion. Therefore, higher temperature generally leads to increased diffusivity of the solute in the liquid solvent. Step 4: Relate liquid viscosity and diffusivity to the rate of leaching.
The rate of leaching is often limited by the mass transfer of the solute from the solid surface into the bulk liquid. Both liquid viscosity and diffusivity play a significant role in this mass transfer process: Viscosity: Lower liquid viscosity facilitates better movement of the solvent around the solid particles and enhances convective mass transfer, bringing fresh solvent into contact with the solid and carrying away the dissolved solute. Diffusivity: Higher diffusivity allows the dissolved solute molecules to move more quickly away from the solid-liquid interface into the bulk liquid, reducing the concentration gradient near the surface and promoting further dissolution. Therefore, decreased liquid viscosity and increased diffusivity both contribute to an increased rate of leaching at higher temperatures. Step 5: Evaluate the given options.
Option 1 (Increased liquid viscosity and decreased diffusivity): This would lead to a decreased rate of leaching, which contradicts the statement.
Option 2 (Increased liquid viscosity and increased diffusivity): Increased viscosity would hinder leaching, so this is incorrect.
Option 3 (Decreased liquid viscosity and decreased diffusivity): Decreased diffusivity would hinder leaching, so this is incorrect.
Option 4 (Decreased liquid viscosity and increased diffusivity): This aligns with the effects of higher temperature on these properties and their positive impact on the leaching rate. Step 6: Select the correct answer.
Higher temperature increases the rate of leaching in a solid-liquid system due to decreased liquid viscosity and increased diffusivity.